Preparation and properties of sulfonated poly(phenylene arylene)/sulfonated polyimide (SPA/SPI) blend membranes for polymer electrolyte membrane fuel cell applications

https://doi.org/10.1016/j.memsci.2011.01.054Get rights and content

Abstract

A novel series of sulfonated poly(phenylene arylene)s (SPAs) were successfully synthesized from 2,5-dichloro-3′-sulfobenzophenone (DCSB) and 2,2′-bis[4-(4-chlorobenzoyl)]phenoxyl perfluoropropane (BCPPF) by Ni(0)-catalyzed coupling polymerization, and subsequently used as the start materials to prepare blend membranes with sulfonated polyimide (SPI) for polymer electrolyte membrane fuel cell (PEMFC) applications. The miscible structure of the prepared blend membranes was confirmed by scanning electron microscopy (SEM). The properties required for a PEM such as ion exchange capacity, water uptake, solution uptake and dimensional change and/or in water or methanol solutions, proton conductivity and hydrolytic stability were investigated in details. The obtained blend SPA/SPI membranes showed rather high proton conductivities in both the plane and the thickness direction of the membrane. They exhibited slightly anisotropic proton conductivity in water with σ⊥/∥ values in the range of 0.85–0.90, irrespective of the introduction of SPI (σ⊥/∥ = 0.73). They also showed significantly enhanced stability in water and methanol solutions compared with the corresponding SPA membranes. All the results indicated that this type of blend membranes were quite promising candidates for PEMFC applications.

Research highlights

► We synthesized a series of blend membranes based on a novel type of sulfonated poly(phenylene arylene)s (SPAs) and sulfonated polyimide (SPI). ► The performances of the blend membranes were significantly improved comparing with either pure SPA or pure SPI membranes. ► The anisotropic proton conductivity ratio (σ⊥/∥) of the membranes in 60 °C water was in the range of 0.85–0.90, closing to that of Nafion 112 and being higher than that of SPI.

Introduction

Over the past decades, a substantial body of research has been devoted to developing novel proton exchange membranes (PEMs) as the alternatives of the state-of-the-art perfluorosulfonic acid membranes (i.e., Nafion of DuPont) [1], [2], [3]. Extensive works have proved that ionomers derived from hydrocarbon aromatic polymers are promising PEM candidates for fuel cell applications, including sulfonated poly(ether ether ketone)s (SPEEKs) [4], [5], [6], sulfonated polyimides (SPIs) [7], [8], [9], polyphosphazenes [10], [11], polybenzimidazoles [12], [13], [14], sulfonated polyphenylenes [15], [16], [17], [18] and so on.

Among them, sulfonated poly(p-phenylene)s (SPPs) or sulfonated poly(phenylene arylene)s (SPAs) derived from sulfonated and non-sulfonated bis(aryl halide)s through Ni(0)-catalyzed coupling reaction are the most recent developed high performance PEM materials. They have rigid rod structure, different from almost all other types of hydrocarbon aromatic polymers, which makes it possible to obtain PEMs with much high IEC and the consequent high proton conductivities. For example, the SPP derivatives synthesized from 2,5-dichloro-4-(4′-sulfophenoxy)benzophenone showed up to 2 orders of magnitude higher proton conductivity comparing to its SPEEK structural isomers at the same condition, except for one more ether bridge in the latter [19]. However, the rigid structure and high IEC level also lead to unfavorable mechanical deterioration and lowering tolerance towards water and methanol solutions of the resulting PEMs. Therefore, it is necessary to make some modifications to improve performance of this kind of PEMs. Lately, our group introduced naphthalimide units into the polymer backbone and synthesized a series of poly(sulfonated phenylene)-block-polyimide copolymers successfully [20]. By the introduction of naphthalimide units as the hydrophobic component, the obtained PEMs exhibited hydrophilic–hydrophobic nanophase-separated morphology, largely enhanced hydrolytic stability and high proton conductivity. Unfortunately, it was difficult to get copolymers with high molecular weight mainly due to the poor solubility of the polynaphthalimide oligomers.

Generally, besides the modifications at chemical structure level as mentioned above, modifications could also be performed at polymer level including blending, compositing and hybridation to prepare high performance PEMs, of which blending is considered to be one of the affective strategies. Recently, many works have been done for the blend PEMs, including: (1) Nafion and hydrocarbon polymers, such as SPEEK, nitrated SPEEK [21], [22]; (2) sulfonated or phosphonated acidic polymers with benzimidazole or pyridine-containing basic polymers [23]. However, undesirable heterogeneous morphology was observed for resulting blend membranes due to the incompatibility of polymers from different types as well as acid–base interactions. On the other hand, blend membranes with miscible structure as reported by Yi et al. based on PES/SPI [24], by Min and Kim of SPEEK/PEI blend membranes [25], and by Kallitsis et al. from polybenzimidazole/sulfonated polysulfone [26], exhibited an inevitable sharply decrease of IEC and proton conductivity because of the introduction of unsulfonated components. To obtain blend membranes with miscible structure without sacrificing the IEC and proton conductivity, we have developed a blend system based on two sulfonated polymers of cross-linked sulfonated poly(arylene ether sulfone) (cSPAES) and SPI [27]. As reported previously, the SPI derived from BAPBDS, BAPB and NTDA with IEC of 1.89 mequiv./g showed good mechanical properties, proton conductivity and high tolerance to water and methanol solutions [28]. As a result, the obtained cSPAES/SPI blend membranes showed rather improved dimensional stability in water and methanol solutions.

It indicated that the introduction of SPI into blend system is a facile and effective approach to get high performance PEMs. In this study, we prepared a series of novel blend membranes from this type of SPA and SPI copolymers. The properties required for a PEM material such as water uptake, solution uptake in methanol solutions, dimensional and water stability, in-plane and through-plane proton conductivities were investigated in details.

Section snippets

Materials

2,5-Dichlorobenzoic acid (Wuhan Fuchi Biological Technology Chemical, China), benzene (Guangdong Guanghua Chemical Factory, China), fluorobenzene (Zhejiang Rainful Chemical, China), p-chlorobenzoyl chloride (Jintan Lanling Chemicals, China), 4,4′-(hexafluoroisopropylidene)diphenol (Zhejiang Norchain Technology Development Co., Ltd, China) and fuming sulfuric acid (SO3, 50%) (Shanghai Zhenhua Chemical, China) were used as received. 2,5-Dichlorobenzophenone was prepared by an AlCl3 catalytic

Results and discussion

In this study, the nomenclature of the blend membranes is as follows: SPA(x/y)-SPI-a/b, where x/y refers to the molar ratio of the sulfonated moiety to the nonsulfonated moiety in SPA, and a/b refers to the weight ratio of SPA to SPI.

Conclusion

A series of novel blend membranes were successfully prepared from two types of sulfonated hydrocarbon aromatic polymers of SPA and SPI. By solution casting method, all the prepared blend membranes were ductile, transparent and exhibited miscible morphology. By the introduction of SPI, the water uptake, solution uptake and dimensional change either in water or in methanol solutions were significantly reduced compared with the SPA membrane. For example, the membrane of SPA(3/1)-SPI-5/5 with the

Acknowledgements

This work was financial supported by Basic Research Program of JiangSu province of China (BK2010482), NUST research funding (grant no. 2010ZDJH03) and Excellent Project of Zijin Star of Nanjing University of Science and Technology (2010). The authors would like to thank Prof. Hongyu Zhang for the FTIR testing and analysis, and Prof. Yi Cheng for the TGA and DSC testing.

References (38)

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